Multi-junction solar cell device

ABSTRACT

A multi-junction solar cell device ( 10 ) is provided. The multi-junction solar cell device ( 10 ) comprises either two or three active solar cells connected in series in a monolithic structure. The multi-junction device ( 10 ) comprises a bottom active cell ( 20 ) having a single-crystal silicon substrate base and an emitter layer ( 23 ). The multi-junction device ( 10 ) further comprises one or two subsequent active cells each having a base layer ( 32 ) and an emitter layer ( 23 ) with interconnecting tunnel junctions between each active cell. At least one layer that forms each of the top and middle active cells is composed of a single-crystal semiconductor alloy that is substantially lattice-matched to the silicon substrate ( 22 ). The polarity of the active p-n junction cells is either p-on-n or n-on-p. The present invention further includes a method for substantially lattice matching single-crystal III-V semiconductor layers with the silicon substrate ( 22 ) by including boron and/or nitrogen in the chemical structure of these layers.

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government has rights in this invention underContract No. DE-AC36-99G010337 between the U.S. Department of Energy andthe National Renewable Energy Laboratory, a Division of Midwest ResearchInstitute.

TECHNICAL FIELD

[0002] This invention relates generally to a multi-junction solar celldevice and, more particularly, it relates to a multi-junction solar celldevice containing III-V layers grown lattice-matched on siliconsubstrates.

BACKGROUND ART

[0003] Solar photovoltaic devices, i.e., solar cells, are devicescapable of converting solar radiation into usable electrical energy. Theenergy conversion occurs as the result of what is known as thephotovoltaic-effect which occurs in a cell composed of a p-typesemiconductor layer adjacent to an n-type semiconductor layer, hereafterreferred to as a p-n junction cell. Solar radiation impinging on a solarcell and absorbed by an active region of semiconductor materialgenerates electricity.

[0004] Multi-junction solar cells may be more efficient thansingle-junction solar cells if properly designed. One such design isdescribed in U.S. Pat. No. 5,223,043 issued to Olson et al. Importantconsiderations to achieve high efficiency energy conversion include thefollowing: a) high quality crystalline layers; b) appropriate choice ofjunction band-gaps based on the impinging solar spectrum; c) tunneljunction interconnects between p-n junctions; d) appropriate choice oflayer thicknesses to achieve a current-matched structure; and e)passivating layers, such as back-surface-field layers or window layers,to reduce losses. In the past, high-efficiency III-V semiconductormulti-junction solar cells have been grown on GaAs, InP, and Gesubstrates, but silicon substrates have been found advantageous for costand mechanical robustness reasons.

[0005] Alloys containing the atoms (AlGaIn)(PAsSb) are examples of III-Vsemiconductors, so named because their constituent elements come fromthe columns IIIb and Vb of the periodic table. In the past, solar cellsconsisting of high-quality, single-crystal layers of (AlGaIn)(PAsSb)semiconductor alloys with a large range of optical properties have beengrown on GaAs, InP, and Ge substrates because these alloys can befabricated with compositions such that the crystal lattice parameter andcrystal symmetry match that of the underlying substrate. This“lattice-matching” condition results in epitaxial layers with minimalstrain, few defects and thus superior electrical properties.Unfortunately, the set of semiconductors alloys (AlGaIn)(PAasSb) cannotbe lattice-matched to silicon for any composition.

[0006] In the past, many investigators have attempted to grow m-V solarcells on single-crystal silicon substrates. Blakeslee et al. (U.S. Pat.No. 4,278,474), Umeno et al. (U.S. Pat. No. 4,963,508), and Ringel etal. (U.S. Pat. No. 5,571,339) have all disclosed lattice-mismatchedIII-V solar cell devices grown on silicon substrates usingstrain-relieving buffer layers. But because these III-V solar celldesigns are not lattice-matched to the underlying silicon, problems withhigh defect densities in the III-V semiconductor layers have preventedsuch solar cell designs from achieving efficiencies as high as those onGaAs or Ge substrates.

[0007] The addition of small amounts of boron (B) and/or nitrogen (N) tothe more standard III-V alloys does allow for compositionslattice-matched to silicon to be reached. For example,GaN_(x)P_(1-x-y)As_(y) is lattice-matched to silicon for 0.022<x<0.194and y=4.6x−0.09. The ability to fabricate these semiconductor alloyswith nitrogen or boron concentrations greater than about 0.1% has onlyrecently been discovered and the achievable compositions and theirproperties are under current investigation.

[0008] In the recent past, GaN_(x)P_(1-x), GaIn_(y)N_(x)P_(1-x), andGaN_(x)P_(1-x-y)As_(y) have been grown on Gap and Si substrates forlight emitting applications. GaN_(x)P_(1-x) has also been shown to havea direct (or direct-like) band gap. B_(x)Ga_(1-x-y)In_(y)As has beengrown on GaAs, but would require considerably greater concentrations ofboron to be lattice-matched to silicon. B_(x)Ga_(1-x)P has not beenattempted but would have a much better chance to be lattice-matched withsilicon than B_(x)Ga_(1-x-y)In_(y)As. All of these III-V semiconductorshave typically been grown using metal-organic vapor phase epitaxy(MOVPE), molecular beam epitaxy (MBE), and similar techniques.

DISCLOSURE OF THE INVENTION

[0009] The present invention is a multi-junction solar cell device. Themulti-junction solar cell device comprises either two or three activesolar cells connected in series in a monolithic structure. Themulti-junction device comprises a bottom active cell having asingle-crystal silicon substrate base and an emitter layer. Themulti-junction device further comprises one or two subsequent activecells each having a base layer and an emitter layer with interconnectingtunnel junctions between each active cell. At least one layer that formseach of the top and middle active cells is composed of a single-crystalIII-V semiconductor alloy that is substantially lattice-matched to thesilicon substrate. The polarity of the active p-n junction cells iseither p-on-n or n-on-p.

[0010] The present invention further includes a method for substantiallylattice matching an active m-V solar cell or cells with an activesilicon solar cell formed from a silicon substrate in a multi-junctionsolar cell device. The method comprises forming the bottom active cellfrom a silicon substrate, and forming the top active cell or cells withat least one III-V semiconductor layer which contains boron and/ornitrogen. The general composition of the III-V semiconductor layer,B_(x)Al_(y)Ga_(1-x-y-z)In_(z)N_(s)P_(1-s-t-w)As_(t)Sb_(w), hereafterreferred to as (BAlGaIn)(NPAsSb), can be lattice matched to a siliconsubstrate only when boron and/or nitrogen compositions are greater thanzero. The quaternary alloys:

[0011] (Ga N_(0.02)P_(0.98))_(1-x)(Ga N_(0.19)As_(0.81))_(x), hereafterreferred to as GaNPAs; and

[0012] (Ga N_(0.02)P_(0.98))_(1-x)(In N_(0.47)P_(0.53))_(x), hereafterreferred to as GaInNP; and

[0013] (B_(0.25)Ga_(0.75)As)_(1-x)(B_(0.02)Ga_(0.98)P)_(x), hereafterreferred to as BGaPAs,

[0014] which are substantially lattice matched to silicon at roomtemperature for 0<x<I make up a subset of the potential list of alloysof the general form (BAlGaIn)(NPAsSb) that are substantially latticematched to silicon. The quaternary alloys listed above are the mostlikely alloys to be used in the present invention due to their relativesimplicity in composition and their useful range of band gaps.

[0015] The lattice matching condition is temperature dependent becausethe thermal expansion coefficient of silicon is different from that ofIII-V semiconductors. Since these III-V semiconductor layers aretypically grown at elevated temperatures, it may be more beneficial tolattice match the III-V semiconductor layers to silicon substrates atgrowth temperature rather than room temperature. When cooled to roomtemperature, III-V semiconductor layers that have been lattice matchedat growth temperature will be slightly lattice mismatched, but shouldalso be considered “substantially lattice matched.”

[0016] In addition, GaP and Al_(x)Ga_(1-x)P are only slightly latticemismatched with silicon substrates, but it may be argued that they arenot “substantially lattice-matched with silicon.”While their high bandgaps and slight lattice mismatch with silicon do not allow them to beuseful as thicker light absorbing layers in the present invention,relatively thin layers of GaP and AlGaP within the structure of thepresent invention do not significantly affect the degree to which therelatively thick active solar cells on silicon substrates are strained.Thus, the active cell which contains a relatively thin GaP or AlGaPlayer does not develop strain-related defects and the entire active cellis considered substantially lattice-matched with silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in and form apart of the specification, illustrate the preferred embodiments of thepresent invention, and together with the descriptions serve to explainthe principles of the invention.

[0018] In the Drawings:

[0019]FIG. 1 is a sectional view illustrating a two-junction solar celldevice, constructed in accordance with the present invention; and

[0020]FIG. 2 is a sectional view illustrating a three-junction solarcell device, constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] As illustrated in FIGS. 1 and 2, the present invention is amulti-junction solar cell device, indicated generally in itstwo-junction version as 10 and in its three-junction version as 100,having lattice-matched (BAlGaIn)(NPAsSb) alloys grown on silicon. Thesolar cell device 10 rivals the efficiencies of high-efficiency cells onGaAs or Ge, with significant cost savings and improvements in mechanicalstability.

[0022] As illustrated in FIG. 1, the two-junction version of the solarcell device 10 of the present invention includes a single-crystalsilicon substrate 22. The two-junction device 10 comprises a bottomactive cell 20, a top active cell 30, and an interconnecting tunneljunction layer 50. The bottom active cell 20 comprises at least a p-type(or n-type) base layer formed within the silicon substrate 22 and ann-type (or p-type) emitter layer 23 forming a p-n junction. The topactive cell 30 comprises at least a p-type (or n-type) base layer 32 andan n-type (or p-type) emitter layer 33 also forming a p-n junction. Thetop active cell 30 comprises at least one nitrogen and/or boroncontaining a m-V semiconductor absorbing layer with the generalcomposition of (BAlGaIn)(NPAsSb) which has a direct (or direct-like)band gap of approximately 1.6 eV to approximately 1.8 eV therebyoptimizing the efficiency of the entire multi-junction solar cell device10. Si, GaP, AlGaP, or various compositions of (BAlGaIn)(NPAsSb) whichare substantially lattice-matched to the silicon substrate 22 would beused in the bottom emitter layer 23, the top emitter layer 33 and thetop base layer 32. Under solar radiation both the top active cell 30 andbottom active cell 20 convert the absorbed portion of the solar spectruminto electrical energy. The tunnel junction 50 is used to facilitate theflow of photogenerated electrical current between the bottom active cell20 and the top active cell 30. The tunnel junction 50 may take a numberof forms to provide a thin layer 50 of materials that allows current topass between cells 20 and 30 without generating a voltage drop largeenough to significantly decrease the conversion efficiency of the solarcell device 10, and that preserves the lattice-matching between cells 20and 30. The photogenerated electrical energy is used or stored in anexternal circuit connected to the metal contacts 70 and 72. The topactive cell 30 and bottom active cell 20 may also contain passivatinglayers, commonly referred to as back-surface-field or window layers, tominimize electrical losses. The use of these passivating layers iscommonly understood among those skilled in the art.

[0023] In a preferred embodiment of the present invention describedabove, the compositions of (BAlGaIn)(NPAsSb) substantially latticematched to the silicon substrate would be chosen from the group ofquaternary alloys; GaNPAs, GaInNP, or BGaPAs specified previously. Thefollowing is a detailed description of a preferred embodiment toillustrate the spirit of the two-junction device 10 of the presentinvention as illustrated in FIG. 1. It should be noted, as understood bya person skilled in the art, that other embodiments of the device 10 arewithin the scope of the present invention

[0024] In this example, III-V semiconductors are grown on asingle-crystal p-type silicon wafer 22 by MOVPE. Source materials wouldinclude triethylgallium, phosphine, tertiary-butyl arsine, anddimethylhydrazine. Growth temperatures would be between 600°-700° C.

[0025] The first layer deposited on the silicon substrate 22 would be awindow layer consisting of 0.1 μm GaP doped highly n-type with Se fromhydrogen selenide. Some of the phosphorus from this layer would diffuseinto the silicon substrate during growth forming a thin n-type emitterlayer 23 within the silicon substrate. Next, an 0.05 μm thick GaP layerdoped highly p-type with carbon from CCl₄ together with the n-type GaPwindow layer would form the tunnel junction 50. A p-type Zn-dopedback-surface-field for the top active cell 30 composed of 0.1 μm GaPwould then be deposited on the tunnel junction 50. A 1.0 μmGaN_(0.07)As_(0.23)P_(0.70) top base layer 32 with a band gap ofapproximately 1.65 eV would serve as the primary absorbing layer of thetop active cell 30. The GaN_(0.07)As_(0.23)P_(0.70) base 32 is notintentionally doped but has a background p-type doping level ofapproximately 1×10¹⁶ cm³. The top emitter 33 is composed of 0.1 μmSe-doped GaP. A 0.1 μm thick highly Se-doped GaAs contact layer isdeposited on the top emitter 33. This GaAs contact layer is removedexcept beneath the metal grid 70 after the Au/Sn/Au metal grids 70 aredeposited by vacuum evaporation and standard photolithographytechniques. A gold back-side contact 72 is also evaporated on the backof the device. A broadband two layer anti-reflective coating is finallydeposited on the

[0026] front of the device. The thickness and composition of theGaN_(0.07)As_(0.23)P_(0.70) top base layer 32 would be adjusted slightlyto achieve a current-matched structure to optimize the efficiency of theentire device 10.

[0027] As illustrated in FIG. 2, the three-junction version of the solarcell device 100 of the present invention includes a single-crystalsilicon substrate 122. The three-junction device 100 comprises a bottomactive cell 120, a middle active cell 140, a top active cell 130, andtwo interconnecting tunnel junction layers 150 and 160. The bottomactive cell 120 comprises at least a p-type (or n-type) base layer 122formed within the silicon substrate 122 and an n-type (or p-type)emitter layer 123 forming a p-n junction. The top active cell 130comprises at least a p-type (or n-type) base layer 132 and an n-type (orp-type) emitter layer 133 also forming a p-n junction. The top activecell 130 comprises at least one nitrogen and/or boron containing a m-vsemiconductor absorbing layer with the general composition of(BAlGaIn)(NPAsSb) which has a direct (or direct-like) band gap ofapproximately 1.8 eV to approximately 2.0 eV thereby optimizing theefficiency of the entire multi-junction solar cell device 100. Themiddle active cell 140 comprises at least a p-type (or n-type) baselayer 142 and an n-type (or p-type) emitter layer 143 also forming a p-njunction. The middle active cell 140 comprises at least one nitrogenand/or boron containing III-V semiconductor absorbing layer with thegeneral composition of (BAlGaIn)(NPAsSb) which has a direct (ordirect-like) band gap of approximately 1.4 eV to approximately 1.5 eVthereby optimizing the efficiency of the entire multi-junction solarcell device 100. Si, GaP, AlGaP, or various compositions of(BAlGaIn)(NPAsSb) which are substantially lattice-matched to the siliconsubstrate 122 would be used in the bottom emitter layer 123, the middleemitter layer 143, the middle base layer 142, the top emitter layer 133and the top base layer 132. Under solar radiation the three active cells120, 130, and 140 each convert the absorbed portion of the solarspectrum into electrical energy. The tunnel junction 150 is used tofacilitate the flow of photogenerated electrical current between theactive cells 120 and 140. The tunnel junction 160 is used to facilitatethe flow of photogenerated electrical current between the active cells140 and 130. The tunnel junctions 150 and 160 may take a number of formsthat allows current to pass between cells 120, 140, and 130 withoutgenerating a voltage drop large enough to significantly decrease theconversion efficiency of the solar cell device 100, and that preservesthe lattice-matching between cells 120, 140, and 130. The photogeneratedelectrical energy is used or stored in an external circuit connected tothe metal contacts 170 and 172. The active cells 120, 140, and 130 mayalso contain passivating layers, commonly referred to asback-surface-field or window layers, to minimize electrical losses. Theuse of these passivating layers is commonly understood among thoseskilled in the art.

[0028] In a specific embodiment of the three-junction version of thesolar cell device 100 of the present invention, the compositions of(BAlGaIn)(NPAsSb) substantially lattice matched to the silicon substratewould be chosen from the group of quaternary alloys; GaNPAs, GaInNP, orBGaPAs specified previously.

[0029] The foregoing exemplary descriptions and the illustrativepreferred embodiments of the present invention have been explained inthe drawings and described in detail, with varying modifications andalternative embodiments being taught. While the invention has been soshown, described and illustrated, it should be understood by thoseskilled in the art that equivalent changes in form and detail may bemade therein without departing from the true spirit and scope of theinvention, and that the scope of the present invention is to be limitedonly to the claims except as precluded by the prior art. Moreover, theinvention as disclosed herein, may be suitably practiced in the absenceof the specific elements which are disclosed herein.

1. A multi-junction, monolithic solar cell device for converting solarradiation into electrical energy, the multi-junction solar cell devicecomprising: a bottom active cell having a single-crystal siliconsubstrate base and a bottom emitter layers which together form a firstp-n junction; a top active cell having a top base layer and a topemitter layer which together form a second p-n junction, and wherein thetop active cell includes at least one III-V semiconductor layer that issubstantially lattice matched to the silicon substrate; and a tunneljunction layer interposed between the bottom active cell and the topactive cell for facilitating electrical current flow between the bottomactive cell and the top active cell.
 2. The multi-junction solar celldevice of claim 1 wherein the top active cell has at least one layerwith a composition of (BAlGaIn)(NPAsSb) that is substantiallylattice-matched to the silicon substrate.
 3. The multi-junction solarcell device of claim 2 wherein the (BAlGaIn)(NPAsSb) layer in the topactive cell has a direct band-gap being approximately 1.6 eV toapproximately 1.8 eV.
 4. The multi-junction solar cell device of claim 3wherein each of the bottom emitter and the top emitter, comprise a layerof material selected from the group consisting of Si, GaP, AlGaP, and atleast one composition of (BAlGaIn)(NPAsSb) that is substantiallylattice-matched to the silicon substrate, and wherein the top base layercomprises a layer of material selected from the group consisting of Si,GaP, AlGaP, and at least one composition of (BalGaIn)(NPAsSb) that issubstantially lattice-matched to the silicon substrate.
 5. Themulti-junction solar cell device of claim 4 wherein the compositions of(BAlGaIn)(NPAsSb) substantially lattice matched to the silicon substrateare selected from the group of quaternary alloys consisting of GaNPAs,GaInNP, and BGaPAs.
 6. A multi-junction, monolithic solar cell devicefor converting solar radiation into electrical energy, themulti-junction solar cell device comprising: a bottom active cell havinga first p-n junction formed by a single-crystal silicon substrate baseand a bottom emitter layer; a middle active cell having a second p-njunction formed by a middle base layer and a middle emitter layer,wherein the middle active cell contains at least one III-V semiconductorlayer; a top active cell having a third p-n junction formed by a topbase layer and a top emitter layer, wherein the top active cell containsat least one III-V semiconductor layer; a first tunnel junction layerinterposed between the bottom active cell and the middle active cell forfacilitating electrical current flow between the bottom active cell andthe middle active cell; and a second tunnel junction layer interposedbetween the middle active cell and the top active cell for facilitatingelectrical current flow between the middle active cell and the topactive cell; wherein the top active cell and the middle active cell aresubstantially lattice-matched to the silicon substrate.
 7. Themultijunction solar cell device of claim 6 wherein the top active cellhas at least one layer with a composition of (BAlGaIn)(NPAsSb) that issubstantially lattice-matched to the silicon substrate.
 8. Themulti-junction solar cell device of claim 7 wherein the middle activecell has at least one layer with a composition of (BAlGaIn)(NPAsSb) thatis substantially lattice-matched to the silicon substrate.
 9. Themulti-junction solar cell device of claim 8 wherein the(BAlGaIn)(NPAsSb) layer in the top active cell has a direct band-gapbeing approximately 1.8 eV to approximately 2.0 eV.
 10. Themulti-junction solar cell device of claim 9 wherein the(BAlGaIn)(NPAsSb) layer in the middle active cell has a direct band-gapbeing approximately 1.4 eV to approximately 1.5 eV.
 11. Themulti-junction solar cell device of claim 10 wherein each of the bottomemitter, the middle emitter, and the top emitter, comprises a materialselected from the group consisting of Si, GaP, AlGaP, and at least onecomposition of (BAlGaIn)(NPAsSb) that is substantially lattice-matchedto the silicon substrate, and wherein each of the middle base and thetop base comprises a layer of material selected from the groupconsisting of Si, GaP, AlGaP, and at least one composition of(BalGaIn)(NPAsSb) that is substantially lattice-matched to the siliconsubstrate.
 12. The multi-junction solar cell device of claim 11 whereinthe compositions of (BAlGaIn)(NPAsSb) substantially lattice matched tothe silicon substrate are selected from the group of quaternary alloysconsisting of GaNPAs, GaInNP, and BGaPAs.
 13. A method for convertingsolar radiation into electrical energy, the method comprising: forming afirst p-n junction with a bottom active cell having a single-crystalsilicon substrate base and a bottom emitter layer; forming a second p-njunction with a top active cell having a top base layer and a topemitter, the top active cell containing at least one III-V semiconductorlayer; facilitating electrical current flow between the bottom activecell and the top active cell; substantially lattice matching the topactive cell to the silicon substrate.
 14. The method of claim 13 whereinthe top active cell layer has at least one layer with a composition of(BAlGaIn)(NPAsSb) that is substantially lattice-matched to the siliconsubstrate.
 15. The method of claim 14 wherein the (BAlGaIn)(NPAsSb)layer in the top active cell has a direct band-gap being approximately1.6 eV to approximately 1.8 eV.
 16. The method of claim 15 wherein eachof the bottom emitter and the top emitter, comprises a layer of materialselected from the group consisting of Si, GaP, AlGaP, and at least onecomposition of (BAlGaIn)(NPAsSb) that is substantially lattice-matchedto the silicon substrate and wherein the top base comprises a layer ofmaterial selected from the group consisting of Si, GaP, AlGaP, and atleast one composition of (BalGaIn)(NPAsSb) that is substantiallylattice-matched to the silicon substrate.
 17. The method of claim 16wherein the compositions of (BAlGaIn)(NPAsSb) are substantially latticematched to the silicon substrate and are selected from the group ofquaternary alloys consisting of GaNPAs, GaInNP, and BGaPAs.
 18. A methodfor converting solar radiation into electrical energy, the methodcomprising: forming a first p-n junction with a bottom active cellhaving a single-crystal silicon substrate base and a bottom emitterlayer; forming a second p-n junction with a middle active cell having amiddle base layer and a middle emitter layer, the middle active cellcontaining at least one III-V semiconductor layer; forming a third p-njunction with a top active cell having a top base layer and a topemitter layer, the top active cell containing at least one III-Vsemiconductor layer; facilitating electrical current flow between thebottom active cell and the middle active cell; facilitating electricalcurrent flow between the middle active cell and the top active cell; andsubstantially lattice matching the top active cell and the middle activecell.
 19. The method of claim 18 wherein the top active cell has atleast one layer with a composition of (BAlGaIn)(NPAsSb) that issubstantially lattice-matched to the silicon substrate.
 20. The methodof claim 19 wherein the middle active cell has at least one layer with acomposition of (BAlGaIn)(NPAsSb) that is substantially lattice-matchedto the silicon substrate.
 21. The method of claim 20 wherein the(BAlGaIn)(NPAsSb) layer in the top active cell has a direct band-gapbeing approximately 1.8 eV to approximately 2.0 eV.
 22. The method ofclaim 21 wherein the (BAlGaIn)(NPAsSb) layer in the middle active cellhas a direct band-gap being approximately 1.4 eV to approximately 1.5eV.
 23. The method of claim 22 wherein each of the bottom emitter, themiddle emitter, and the top emitter comprises a layer of materialselected from the group consisting of Si, GaP, AlGaP, and at least onecomposition of (BalGaIn)(NPAsSb) that is substantially lattice-matchedto the silicon substrate, and each of the middle base and the top basecomprises a layer of material selected from the group consisting of Si,GaP, AlGaP, and at least one composition of (BAlGaIn)(NPAsSb) that issubstantially lattice-matched to the silicon substrate.
 24. The methodof claim 23 wherein the compositions of (BAlGaIn)(NPAsSb) substantiallylattice matched to the silicon substrate are selected from the group ofquaternary alloys consisting of GaNPAs, GaInNP, and BGaPAs.
 25. Aphotovoltaic device, comprising: a single-crystal silicon substratecomprising a first base layer; a first emitter layer forming a first p-njunction with the base layer; a second base layer; a second emitterlayer forming a second p-n junction with the second base layer; and afirst tunnel junction layer between the first emitter layer and secondbase layer for facilitating electrical current flow therebetween,wherein at least one of the second base layer and second emitter layercomprise a layer of III-V semiconductor, and the first base layer, firstemitter layer, tunnel junction layer, second base layer and secondemitter layer are substantially lattice-matched with each other.
 26. Thephotovoltaic device of claim 25, wherein the first emitter layercomprises a layer in the silicon substrate.
 27. The photovoltaic deviceof claim 25, wherein the III-V semiconductor comprises(BalGaIn)(NPAsSb).
 28. The photovoltaic device of claim 27, wherein thelayer of (BAlGaIn)(NPAsSb) has a direct band-gap being approximately 1.6eV to approximately 1.8 eV.
 29. The photovoltaic device of claim 25,wherein the second base layer comprises a layer of (BalGaIn)(NPAsSb).30. The photovoltaic device of claim 27, wherein the Ill-V semiconductorcomprises at least one of (Ga N_(0.02)P_(0.98))_(1-x)(GaN_(0.19)As_(0.81))_(x), (Ga N_(0.02)P_(0.98))_(1-x)(InN_(0.47)P_(0.53))_(x) and(B_(0.25)Ga_(0.75)As)_(1-x)(B_(0.02)Ga_(0.98)P)_(X), where 0<x<1. 31.The photovoltaic device of claim 27, wherein the III-V semiconductorcomprises GaN_(x)P_(1-x-y)As_(y), where 0.022<x<0.194 and y=4.6x−0.09.32. The photovoltaic device of claim 25, further comprising: a thirdbase layer; a third emitter layer forming a third p-n junction with thethird base layer; and a second tunnel junction layer between the secondemitter layer and third base layer for facilitating electrical currentflow therebetween, wherein at least one of the third base layer andthird emitter layer comprises a layer of III-V semiconductor, and thefirst base layer, first emitter layer, first tunnel junction layer,second base layer, second emitter layer, second tunnel junction layer,third base layer and third emitter layer are substantiallylattice-matched with each other.
 33. The photovoltaic device of claim32, wherein the III-V semiconductor in at least one of the second baselayer and second emitter layer has a band-gap of about 1.4 to about 1.5eV, and the III-V semiconductor in at least one of the third base layerand third emitter layer has a band-gap of about 1.8 to about 2.0 eV. 34.The photovoltaic device of claim 26, wherein the first base layercomprise a p-type layer of the silicon substrate, the first emitterlayer comprises an n-type layer of the silicon substrate, the secondbase layer comprises a layer of p-type GaNAsP, and the second emitterlayer comprises a layer of n-type GaP.